The present invention relates to wireless communication, and more particularly to wireless communication utilizing adaptive hybrid automatic repeat request (HARQ) protocols.
The ultimate purpose of a communication system is to transmit information from an information source to a destination over a communication channel. In wireless communication systems noise and multipath fading cause bit errors to occur during transmission. A variety of error control techniques are used to combat transmission errors and reduce bit errors. Most error control techniques introduce controlled redundancy into the information transmitted over the communication channel that can be used at the destination to detect and possibly correct bit errors that may occur during transmission. Two commonly used error control techniques are automatic repeat request (ARQ) and forward error correction (FEC).
The basic principle underlying ARQ is to add redundant bits or check bits to a message that allows detection of errors at the receiver. If the receiver detects errors in the received message, the receiver can request a repeat transmission of the message, either by sending an explicit request for retransmission or by not acknowledging receipt of the message. ARQ is simple and achieves reasonable throughput when the error rate is low. Throughput diminishes, however, as the error rate increases because of the need to resend more data.
FEC uses error-correcting codes to combat errors by adding redundancy to information before it is transmitted. The added redundancy enables the receiver to detect and correct most errors that occur during transmission. Examples of FEC codes include block codes, convolutional codes, and turbo codes.
Hybrid ARQ (HARQ) is another error control technique that combines ARQ and FEC. HARQ protocols are generally adopted for high-speed down-link packet access (HSDPA) channels to further ensure robustness against link adaptation errors. Using HARQ, messages are coded twice using an inner code and an outer code. The outer code may, for example, comprise a CRC code that is appended to the information bits prior to transmission to form a protected message. The protected message is then coded using a convolutional code or turbo code. Both the information bits and CRC bits are coded. The coded message is then transmitted to a receiving terminal, which decodes the message and performs a CRC check. If the number of errors in the message is within the capabilities of the error correction code, the errors will be corrected without the need for retransmission. Only if the number of errors in the message exceeds the capabilities of the error correcting code will retransmission be requested.
Chase Combining (CC) and incremental redundancy (IR) are two commonly used HARQ protocols. Chase combining is a bit repetition protocol. If an initial transmission is received with one or more errors at a receiver, the corrupted data packet is stored in memory and retransmission is requested. The transmitter repeats the same message. The receiver combines the retransmitted data with the corrupted data stored in memory prior to decoding. CC provides time diversity and gain by accumulating bit energies. IR provides additional coding gains by transmitting additional parity bits with each retransmission. The initial transmission includes systematic bits and a punctured version of the parity bits output by the FEC encoder. With each retransmission, additional parity bits are transmitted. Thus, IR reduces the effective code rate with each retransmission.